Extracts of Desulfovibrio vulgaris were tested for their ability to synthesize cysteine from sulfide and O-acetyl-L-serine. Cysteine was found to be rapidly synthesized to determine if the sulfur moiety of cysteine could be provided by bisulfite, this compound was incubated with extracts under a hydrogen atmosphere. The bisulfite was reduced to sulfide which was incorporated into cysteine. This demonstrated that extracts of D. vulgaris could reduce bisulfite by a dissimilatory pathway and assimilate the sulfide into an organic compound.